1. Field of the Invention
The present invention relates to a plasma display apparatus and a driving method thereof.
2. Description of the Background Art
In general, a plasma display apparatus comprises a plasma display panel where one unit cell is provided at a space between barrier ribs formed between a front substrate and a rear substrate. Main discharge gas such as neon (Ne), helium (He) or a mixture (He+Ne) of neon and helium and inert gas containing a small amount of xenon (Xe) are filled in each cell. When discharge is performed using high frequency voltage, the inert gas generates vacuum ultraviolet rays and phosphors provided between the barrier ribs are emitted, thereby realizing an image.
The plasma display panel is attracting attention as a next generation display due to its slimness and lightweighting.
FIG. 1 illustrates a structure of a conventional plasma display panel.
As shown in FIG. 1, a plasma display panel comprises a front substrate 100 and a rear substrate 110. The front substrate 100 has a plurality of sustain electrode pairs arranged with a scan electrode 102 and a sustain electrode 103 each paired and formed on a front glass 101, which is a display surface for displaying the image thereon. The rear substrate 110 has a plurality of address electrodes 113 arranged to intersect with the plurality of sustain electrode pairs on a front glass 111, which is spaced apart in parallel with and attached to the front substrate 100.
The front substrate 100 includes the paired scan electrode 102 and the paired sustain electrode 103 for performing a mutual discharge in one pixel and sustaining emission of light, that is, the paired scan electrode 102 and the paired sustain electrode 103 each having a transparent electrode (a) formed of indium-tin-oxide (ITO) and a bus electrode (b) formed of metal. The scan electrode 102 and the sustain electrode 103 are covered with at least one dielectric layer 104, which controls a discharge current and insulates the paired electrodes. A protective layer 105 is formed of magnesium oxide (MgO) on the dielectric layer 104 to facilitate a discharge condition.
The rear substrate 110 includes stripe-type (or well-type) barrier ribs 112 for forming a plurality of discharge spaces (that is, discharge cells) and arranged in parallel. Also, the rear substrate 110 comprises a plurality of address electrodes 113 arranged in parallel with the barrier ribs 112, and performing an address discharge and generating the vacuum ultraviolet rays. Red (R), green (G), blue (B) phosphors 114 emit visible rays for displaying the image in the address discharge, and are coated over an upper surface of the rear substrate 110. Lower dielectric layer 115 for protecting the address electrode 113 is formed between the address electrode 113 and the phosphor 114.
In the above structured plasma display panel, the discharge cells are formed in matrix in plural, and a driving module having a driving circuit for supplying a predetermined pulse to the discharge cell is connected and driven.
FIG. 2 is a view illustrating a conventional method for expressing the image gray level in a plasma display apparatus.
As shown in FIG. 2, in the conventional method for expressing the image gray level in the plasma display apparatus, one frame is divided into several subfields having the different number times of emission. Each subfield is divided into a reset period (RPD) for initializing all cells, an address period (APD) for selecting a discharged cell, and a sustain period (SPD) for expressing the gray level depending on the number times of discharge. For example, when the image is displayed in 256 gray levels, as shown in FIG. 3, a frame period (16.67 ms) corresponding to a 1/60 second is divided into eight subfields (SF1 to SF8), and each of the eight subfields (SF1 to SF8) is divided into the reset period, the address period, and the sustain period. The reset period and the address period are the same at each subfield. The address discharge for selecting the cell to be discharged is generated by a voltage difference between the address electrode and the scan electrode being the transparent electrode. The sustain period is increased in a ratio of 2n (n=0, 1, 2, 3, 4, 5, 6, 7) at each subfield. Since the sustain period is different at each subfield as described above, the sustain period of each subfield (that is, the number times of sustain discharge) is controlled, thereby expressing the image gray level.
In the meantime, in the conventional plasma display apparatus, as a temperature around the plasma display panel gets higher, erroneous discharge is generated. The erroneous discharge generated when the temperature around the panel is high is called “high temperature erroneous discharge”. Such the high temperature erroneous discharge will be described with reference to FIG. 3.
FIG. 3 illustrates a charge state within a conventional discharge cell.
Referring to FIG. 3, in the conventional plasma display apparatus, as the temperature around the panel gets higher, a recombination ratio between space charges 301 and wall charges 300 within the discharge cell increases and therefore, an absolute amount of the wall charges participating in the discharge decreases, thereby causing the erroneous discharge. The space charges 301 being charges existing in a space within the discharge cell, refer to charges not participating in the discharge unlike the wall charges 300.
For example, the recombination ratio between the space charges 301 and the wall charges 300 increases in the address period to decrease an amount of the wall charges 300 participating in the address discharge, thereby instabilizing the address discharge. In particular, the later addressing is in sequence, the more a time taken to recombine the space charges 301 with the wall charges 300 is sufficiently secured, thereby more instabilizing the address discharge. Therefore, there occurs the high-temperature erroneous discharge where the discharge cell turned-on in the address period is turned off in the sustain period.
Further, as the temperature around the panel gets higher in the sustain period, when a sustain discharge is performed, a speed of creating the space charges 301 is increased in the discharge and accordingly, the recombination ratio of the space charges 301 and the wall charges 300 are increased. Accordingly, there occurs the high-temperature erroneous discharge where after one-time sustain discharge, the wall charges 300 participating in the sustain discharge are decreased in amount by the recombination of the space charges 301 and the wall charges 300, thereby preventing a next sustain discharge.
FIG. 4 illustrates a driving waveform of a conventional plasma display apparatus.
As shown in FIG. 4, the conventional plasma display apparatus is driven with each subfield divided into the reset period for initializing all cells, the address period for selecting the cell to be discharged, the sustain period for sustaining a discharge of the selected cell, and the erasure period for erasing the wall charge within the discharge cell.
Referring to FIG. 4, in the driving waveform of the conventional plasma display apparatus, all address waveforms applied to the address electrodes (X1 to Xn) are applied at the same time “ts” as the scan waveform applied to the scan electrode in the address period. If the address waveform and the scan waveform are applied to the address electrodes (X1 to Xn) and the scan electrode respectively at the same time point, a noise is generated at the waveform applied to the scan electrode and the waveform applied to the sustain electrode.
This noise results from coupling through capacitance of the panel. At a time point when the address waveform abruptly rises, an up noise is generated at the waveform applied to the scan electrode and the sustain electrode, and at a time point when the address waveform abruptly falls, a down noise is generated at the waveform applied to the scan electrode and the sustain electrode. This causes a drawback of instabilizing the address discharge generated in the address period, thereby reducing a driving efficiency of the plasma display panel.